The present invention relates generally to a filter structure as typically used in microfluidic devices and, more particularly, unique structures for a filter having particular use in an ink jet printer system, i.e. increasing fluid flow through a filter by increasing the surface area of the filter.
There is a trade-off in filter design between flow resistance and filter effectiveness especially for small particle size. Microfilters traditionally have a relatively high flow resistance although they offer precise filter sizing with 100 percent particle retention for particle sizes above the pore size of the filter. In thermal ink jet systems, for example, the implication for small enough pore size is that the printing frequency might be limited by the flow through the filter. For various drop sizes and printing frequencies, simple patterns of circular pores are adequate. However, there is a general interest in going to smaller drop sizes, e.g. (requiring a finer filter) and higher frequencies in the order of 15 khz and higher.
In new areas of microfluidics, microfluidic carrying devices and their components are small, typically in the range of 500 microns down to as small as 1 micron, and possibly even smaller. Such microfluidic devices pose difficulties with regards to maintaining and increasing fluid flow through the microscopic componentry, and, especially, when the particular microscopic componentry is connected to macroscopic sources of fluid. Yet such microfluidic devices are important in a wide range of applications that include drug delivery, analytical chemistry, microchemical reactors and synthesis, genetic engineering, and printing technologies including a wide range of ink jet technologies, such as thermal ink jet printing.
A typical thermally actuated drop-on-demand ink jet printing system, for example, uses thermal energy pulses to produce vapor bubbles in an ink-filled channel that expels droplets from the channel nozzles of the printing system""s print head. Such print heads have one or more ink-filled channels communicating at one end with a relatively small ink supply chamber (or reservoir) and having a nozzle at the opposite end. A thermal energy generator, usually a resistor, is located within the channels near the nozzle at a predetermined distance upstream therefrom. The resistors are individually addressed with a current pulse to momentarily vaporize the ink and form a bubble which expels an ink droplet.
Some of these thermal ink jet print heads are formed by mating two silicon substrates. One substrate contains an array of heater elements and associated electronics (and is thus referred to as a heater plate), while the second substrate is a fluid directing portion containing a plurality of nozzle-defining channels and an ink inlet for providing ink from a source to the channels. This substrate is referred to as a channel plate which is typically fabricated by orientation dependent etching methods.
The dimensions of the ink inlets to the die modules, or substrates, are much larger than the ink channels. Hence, it is desirable to provide a filtering mechanism for filtering the ink at some point along the ink flow path from the ink manifold or manifold source to the ink channel or from the ink channel to the nozzle to prevent blockage of the channels by various particles typically carried in the ink. Even though some particles of a certain size do not completely block the channels, they can adversely affect directionality of a droplet expelled from these print heads.
U.S. Pat. No. 4,864,329 to Kneezel et al. discloses a thermal ink jet printhead having a flat filter placed over the inlet thereof by a fabrication process which laminates a wafer size filter to the aligned and bonded wafers containing a plurality of print heads. The individual print heads are obtained by a sectioning operation, which cuts through the two or more bonded wafers and the filter. The filter may be a woven mesh screen or preferably a nickel electroformed screen with predetermined pore size. Electroformed screen filters having pore size which is small enough to filter out particles result in filters which are very thin and subject to breakage during handling or wash steps. Also, the preferred nickel embodiment for a filter is not compatible with certain inks resulting in filter corrosion. Finally, the choice of materials is limited when using this technique. Woven mesh screens are difficult to seal reliably against both the silicon ink inlet and the corresponding opening in the ink manifold. Further, plating with metals such as gold to protect against corrosion is costly. This patent is intended to be incorporated by reference herein in its entirety.
In all cases, conventional microfilters ordinarily suffer from blockage by particles larger than the pore size, and by air bubbles. Conventional microfilters used for thermal ink jet print heads help keep the jetting nozzles and channels free of clogs caused by dirt and air bubbles carried into the printhead from upstream sources such as from the ink supply cartridge. One common failing of all planar microfilters is their relatively high flow resistance and limited surface area for filter pores.
In laser ablated filters, circular holes are laser ablated in a flat planar plastic film, which may then be bonded over the ink inlets of many die at once in a thermal ink jet wafer, as taught in U.S. Pat. No. 6,139,674, to Markham et al. and U.S. patent application Ser. No. 6,199,980, to Fisher et al., both commonly assigned as the present application and both incorporated by reference. However, even when the holes are packed as tightly as possible, the open planar area for typical filter dimensions may be on the order of 40%.
In an ink jet system environment, one of the basic objectives of the embodiments of the present invention is to provide a filter which will prevent particles of a size sufficient to block channels from entering the printhead channels and minimize fluid flow resistance due to the filter along the ink flow path.
It is an object of the present invention to provide a microfluidic filtering device with increased surface area.
According to the present invention, a microfluidic filter has a pleated filter structure having a plurality of pores through the structure. The pleated filter can be either an open loop or a closed loop pleated structure. The pore structure of the pleated filter is formed by laser ablation.
Another embodiment of the present invention is directed to an improved ink jet printhead having an ink inlet in one of its surfaces, a plurality of nozzles, individual channels connecting the nozzles to an internal ink supplying manifold, the manifold being supplied ink through the ink inlet, and selectively addressable heating elements for expelling ink droplets, the improved ink jet printhead comprising a pleated filter having predetermined dimensions with the filter having a plurality of pores. The open loop pleated filter can be bonded within the printhead at the ink inlet or the closed loop pleated filter can be bonded at other points along the ink flow path between the manifold and the nozzle.
Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings.